AQA A-Level Chemistry AS Paper 1, November 2021: Question 2

✅ Correct Answer

Observation: Bright white light / white powder or solid formed.

Equation: Mg(s) + H₂O(g) ➔ MgO(s) + H₂(g)

💡 Key Knowledge

• Magnesium reacts vigorously with steam (much faster than with cold water) to produce magnesium oxide and hydrogen gas.
• State symbols are (s) for solid magnesium and magnesium oxide, (g) for steam, and (g) for hydrogen.

🧠 Exam Technique

When an exam question specifies "including state symbols", always double-check every single species. Forgetting just one state symbol will cost you the mark for the equation!

❌ Common Errors

Students frequently write (l) or (aq) for water, forgetting that the question specifically states steam (which must be g ). Confusing the reaction with cold water ( Mg + 2H₂O ➔ Mg(OH)₂ + H₂ ) is also a frequent mistake.

✅ Correct Answer

M1: Attraction between (lattice of) Mg²⁺ ions (or positive ions / nuclei)...

M2: ...and delocalised electrons.

💡 Key Knowledge

Metallic bonding is defined as the electrostatic attraction between positive metal ions and delocalised electrons. For Group 2 metals like magnesium, each atom contributes two electrons to the sea of delocalised electrons, forming Mg²⁺ ions.

🧠 Exam Technique

To secure both marks, you must explicitly mention both components of metallic bonding: the positive ions (and their charge, Mg²⁺ ) and the delocalised electrons. Using buzzwords like "sea of electrons" is helpful, but naming "delocalised electrons" is essential for M2.

❌ Common Errors

Vague answers stating "bonds between atoms" or describing intermolecular forces / sharing of electrons (covalent bonding) score zero.

✅ Correct Answer

1. (Giant) ionic lattice / lots of Mg²⁺ and Cl⁻ ions.

2. Strong (electrostatic) forces of attraction.

3. Between Mg²⁺ and Cl⁻ ions (oppositely charged ions).

💡 Key Knowledge

Magnesium chloride ( MgCl₂ ) is an ionic compound. Breaking its high melting point requires supplying a large amount of energy to overcome the strong electrostatic forces operating in all directions throughout the giant ionic lattice.

🧠 Exam Technique

Follow a structured template for ionic explanation questions:
1. Identify the structure type (Giant ionic lattice).
2. State the nature of the particles ( Mg²⁺ and Cl⁻ ions).
3. Describe the forces (Strong electrostatic attraction between oppositely charged ions requiring a lot of energy to break).

❌ Common Errors

A major pitfall is mentioning "intermolecular forces" or "molecules" when discussing ionic compounds. MgCl₂ does not exist as discrete molecules, so using molecular terminology loses marking points.

✅ Correct Answer

Indigestion relief / laxative / neutralising excess stomach acid (Allow "Milk of Magnesia").

💡 Key Knowledge

Group 2 hydroxides become more soluble down the group, but magnesium hydroxide is sparingly soluble. Because it is weakly alkaline and safe to ingest in small amounts, it is used medicinally to neutralise excess hydrochloric acid in the stomach without causing dangerous systemic side effects.

🧠 Exam Technique

This is a straightforward recall question worth 1 mark. Keep your answer concise and direct—stating "neutralises stomach acid" or "indigestion remedy" instantly secures the mark.